The present invention relates to fluorescent lamps having metallic electrode guards, and more particularly to a safe improvement in such lamps that reduces lamp power dissipation.
Low pressure mercury vapor discharge lamps having a phosphor layer for emitting light, commonly known as fluorescent lamps, generally have electrodes made of coiled tungsten wire. These electrodes are coated with a material for enhancing the thermionic emission of electrons. During lamp operation tungsten and emitter material can evaporate or sputter from the electrodes and be deposited in the area of the electrodes on the lamp wall in the form of tungsten and tungsten products. This deposition is evident as visible blackening and is a detrimental consequence of lamp operation.
One technique for suppressing the blackening from electrode materials is to partially surround each electrode with a guard. The guard is typically in the form of a closed ring made of a conductive metal strip and positioned surrounding the sides of the electrode. Examples of this structure are shown in U.S. Pat. Nos. 4,032,813 and 4,032,814.
In these patents the conductive metal strip carries a getter such as a mixture of zirconium and aluminum for reducing the quantity of unwanted impurity gasses. Additionally, the metal strip supports a small capsule of mercury needed for normal lamp operation. Finally, the conductive metal strip reduces the lamp's energy consumption.
Both of the patents just mentioned show lamp structure in which the electrical potential of the electrode guard is floating relative to the electrode potential. This is achieved by mechanically supporting it on a mount that is electrically insulated from the supports of the electrode. The floating arrangement of the electrode guard is also mentioned in the text Fluorescent Lamps And Lighting, W. Elenbaas, ed., Sec. 5.3 (1962).
In fluorescent lamps that operate on alternating current the electrodes operate alternately as a cathode and an anode. As discussed in the text Electric Discharge Lamps, John Waymouth, Chapter 4 (MIT 1971), the alternating function of the electrodes requires compromises in the electrode design. Ideally, a fluorescent lamp anode electrode would have a large area to reduce the potential difference between the anode and the plasma within the lamp, known as the "anode fall". The large area, however, would be detrimental to cathode operation of the electrode which requires rapid heating to thermionic emitting temperatures and to avoid sputtering during glow discharge.
It would be desirable to use the prior art cathode guard as part of the electrode when the electrode is operating as an anode. This would increase the effective area of the anode and thereby reduce the anode fall; however, care would have to be taken to avoid degrading this cathode operation of the electrode.
A lamp employing an electrode shield as part of the anode is disclosed in German Democratic Republic Patentschrift 221,881. In that patent a cold-starting, self-heating electrode is connected to a cylindrical concentric auxiliary electrode, having the structure of a shield, through a diode. When the electrode is biased positive to operate as the anode, the diode polarity is effective to establish a conductive path between the self-heating electrode and the auxiliary electrode. Consequently, the effective electrode total area includes the auxiliary electrode area and the anode fall is reduced. When the electrode is biased negative, that is, as a cathode, the diode polarity presents a high impedance path to the auxiliary electrode which is effectively disconnected from the self-heating electrode.
It would be desirable to use the same technique in lamps with other types of electrodes. However, there are practical reasons for not doing so. The use of a diode within the lamp discharge envelope presents severe quality requirements for the component. The diode must be able to withstand an intense ultraviolet flux, and elevated temperatures for the life of the lamp which may exceed 20,000 operating hours. The diode also contributes to the lamp cost and consequently the avoidance of using a diode would be advantageous.
Additionally, if a lamp having a filament electrode has an electrode guard electrically connected to one current lead of the electrode, there is a danger that the other electrode lead might come in contact with the electrode guard. This could occur, for example, if the filament electrode broke and a piece attached to the electrode lead that is normally unconnected to the guard were to flop over and touch the guard. The consequence would be a low resistance connection across the electrode leads, rather than the normal electrode impedance, with an excessive current flow through the lamp ballast electrode heater winding. If the lamp remained in this condition for an extended period of time, damage to the ballast could occur.